The present invention relates to a structure of an information recording medium permitting high density recording and a method for recording information thereon.
In recent years, there have been proposed information recording media such as DVD (Digital Video Disc or Digital Versatile Disc) which permit a large capacity of information exceeding ten and several gigabytes to be recorded in a high density. In order to manufacture a master disc (information recording medium used as a master copy) to fabricate such a high density information recording medium, a master disc recording method using an electron beam has been proposed in place of a mastering technique using a laser beam.
According to the mastering technique for master disc recording using an electron beam, electron beam resist (hereinafter referred to as "EB resist") is applied on an insulating substrate formed of a glass material having light transmittancy, an electron beam having its spot size reduced is directly irradiated upon the EB resist to perform exposure (electron beam exposure), and the exposed part of the EB resist is removed away by development processing to form pits representing information to be recorded.
Then, in order to evaluate the manufactured master disc, a laser beam having a prescribed wavelength and a reduced spot size is allowed to come in from the side of the insulating substrate, light returned from a recording surface is optically detected and an evaluation for example as to whether the pits or the like are appropriately formed on the recording surface is performed.
As described above, in the mastering technique to which the electron beam exposure is applied, the manufactured master disc must be optically evaluated, and therefore an insulating substrate made of a glass material which is transparent to light, in other words, a material which passes light therethrough is used.
When a master disc having such an insulating substrate is recorded using an electron beam, however, charged electrons accumulate in the insulating substrate by the irradiation of the electron beam. Therefore, the electron beam incident to the EB resist is deflected by the charged electrons and the master disc cannot be recorded in a high density.
FIG. 6A through FIG. 6C are diagrams for use in illustration of a mechanism of how the electron beam is deflected in comparison with a semiconductor manufacturing process.
Referring to FIG. 6A, when electron beam exposure is performed in a semiconductor manufacturing process, EB resist is applied on a silicon semiconductor substrate having a semi-insulating characteristic, and the EB resist is directly irradiated with an electron beam for exposure while the silicon semiconductor substrate is set at a ground potential.
If the silicon semiconductor substrate is thus set at the ground potential, charged electrons e.sup.- generated at the time of the incidence of the electron beam to the silicon semiconductor substrate are passed through the silicon semiconductor substrate having the semi-insulating characteristic and removed onto the side of the ground terminal, so that the electron beam comes into the EB resist straightforward without the influence of the charged electrons e.sup.-, and high resolution electron beam exposure is enabled.
In contrast, the mastering technique for recording a master disc using an electron beam, as shown in FIG. 6B, the EB resist is applied on the above described insulating substrate which is transparent to light, and the EB resist is directly irradiated with the electron beam for exposure. In this case, charged electrons e.sup.- generated at the time of incidence of the electron beam to the insulating substrate accumulate in the insulating substrate. Therefore, the electron beam is deflected by the influence of the charged electrons e.sup.-.
Furthermore, if the electron beam exposure is performed as the insulating substrate is set at the ground potential similarly to the semiconductor manufacturing process, charged electrons e.sup.- accumulate in the insulating substrate rather than being removed to the ground terminal side, because the insulating substrate is not conductive, which will deflect the electron beam.
Thus, in the master disc recording to manufacture an optical recording medium, the electron beam is kept from advancing straightforward, and as a result, it is sometimes difficult to achieve desired high density recording.
In order to reduce the accumulation of charged electrons e.sup.- shown in FIG. 6B, an antistatic film is layered on the EB resist as shown in FIG. 6C, and electron beam exposure is attempted in such a state that the antistatic film is set at the ground potential. Also in this case, however, charged electrons e.sup.- accumulate in the insulating substrate, and therefore the electron beam is deflected in the EB resist by the influence of the charged electrons e.sup.-, which impairs high density master disc recording.
Meanwhile, FIG. 7A is a micrograph showing the shape of pits formed when layering EB resist and an antistatic film on the insulating substrate made of a glass material shown in FIG. 6C and then being followed by electron beam exposure, while FIG. 7B is a micrograph showing the shape of pits formed by electron beam exposure using a silicon semiconductor substrate similarly to the semiconductor manufacturing process shown in FIG. 6A. As can be clearly seen from these micrographs, difficulty in performing high density master disc recording using the insulating substrate was confirmed.